AGP Aperture, the mystery continues............

[spirits247]

I'd say I know quite a few things about PC's, but haven't you noticed now and again those grey areas crop up where everyone seems to fling about advice by word of mouth?

And all this advice is constantly conflicting what you've previously read?

Well AGP aperture size is one of those subjects that I just can't get my head around.

You see, I know what AGP aperture size is, I've read all the advice and technical explanations on the internet a thousand times but still can't reach an overall conclusion to grasp this grey area - everyone shouts out something different!

What instigated this thread? Well, I bought a Geforce GTS 64MB DDR to handle a few rounds of online UT. Previously, I had the AGP aperture set to 64MB for my Matrox G200 and this worked well for the GTS. It should, as this size is enough to hold the address space for the onboard video RAM and provide some space for textures if the onboard 64MB is fully utilised.

The thing I don't understand is that when I set AGP aperture to 128MB, UT noticeable accesses the hard drive a lot more and stutters. When I investigated RAM usage, it was A LOT higher with 128MB aperture than 64MB causing higher swap file usage, and this is the problem I can't get my head around.

I was always under the impression that the system would use the video RAM first (in my case 64MB), then switch to using system RAM for textures up to the amount you've specified for the aperture. That's why this setting isn't that important if you have a large amount of video RAM – it’s unlikely it’ll get used anyway.

My experience however, is that this is not the case. System RAM usage goes way up regardless if texture RAM is full or not, because I can't believe UT is maxing out 64MB of onboard memory and filtering that much info into the system RAM. Is something else is going on?????

From my experience, I'd advise people to set the AGP aperture as low as possible without:

a) Hurting benchmark performance
b) Causing game incompatibility

but set it no lower than the memory you have on your card to allow for address space. It seems setting higher apertures just wastes system RAM as it's used regardless if the cards memory is full or not!

Questions:

Just what is the real deal with AGP aperture sizes?

Do you have to reinstall windows to address the aperture correctly after changing it, because I can't believe the system memory usage between these two settings when I have 64MB onboard video RAM. It appears something is not working right.


Thanks for anyone's advice. I love to clear this grey area up.

[Pinky]

The rule of thumb (as I'm sure you're 101% aware...) is Set the AGP Aperture to 1/2 (50%) of your system memory/RAM.

If you raise the aperture size and it swaps more, then that tells me you don't have as much system memory free as you think.

[Aedan]

I don't fully understand all of AGP - I don't profess to. However, simply setting the AGP aperture to the largest size might not be such a good idea for a number of reasons.

Firstly, the AGP aperture appears as a seperate set of addresses, EVEN THOUGH IT'S THE SAME PHYSICAL RAM! If you have 128Mb of RAM, and an aperture size of 128Mb, the system will allocate enough caching to handle 256Mb of RAM, increasing the usage of your RAM, despite the fact you haven't actually got any more RAM.

In Win9x machines, this causes problems, as the virtual address table for 512Mb RAM, coupled with the 128Mb aperture size means that VCache might run out of virtual addresses to use before finishing allocating virtual addresses to hardware. Obviously this is NOT a good state of affairs!

In NT based machines, the caching system can handle far more RAM, but still you'll lose RAM to the increased cache size.

Secondly, if your card has a large amount of RAM, you don't need such a large AGP aperture, as it's less likely to use any. The "rule of thumb" about setting it to half the RAM is invalid anyhow.

You can't have an AGP aperture less than 16Mb, as control structures that go with it are around 12Mb in size. This is why setting it less than 16Mb means you drop out of AGP mode.

Tuning the AGP aperture looks to be pretty badly documented. It looks like you'll have to run your favourite games, and bring the AGP aperture down until it starts to eat into your frame rate. The less memory you have on your card, the quicker this will happen!

AidanII